The present invention relates to a transmittable light-scattering sheet (or film) useful for assuring a high-luminance display of images in a liquid crystal display device (in particular, reflective liquid crystal display device), a method of producing the same, and a reflective liquid crystal display device utilizing the light-scattering sheet.
The liquid crystal display device (LCD) is utilized broadly in the display segments of personal computers, word processors, LC televisions, chronometers, desktop calculators and other electrical and electronic products. Since the liquid crystal by itself does not emit light, a backlight for illuminating the liquid crystal cell from the back side is used to the exclusion of low-luminance applications such as watches and desktop calculators.
Recent years have witnessed advances in the construction of infrastructures for telecommunications systems such as internets and in the network consolidation of information through the computer-communications equipment integration. By network consolidation, the access to information is freed from restrictions as to time and place. For an efficient utilization of such networks, currently, portable information terminals such as PDA (personal digital assistance) have been developed. Moreover, in lieu of notebook-sized personal computers, further-downsized mobile personal computers of reduced thickness and weight are now under development.
Since portability is required of these portable telecommunications equipment, it is necessary to reconcile the need for a longer battery operating time with the need for reduced thickness and downsizing of communications devices. Therefore, displays for use in such portable telecommunications equipment must be of reduced thickness, reduced weight and low power consumption. Particularly for attaining the low power consumption goal, replacing the conventional liquid crystal display device using a backlight, a reflecting type liquid crystal display device is considered to be most promising as a display for brightening the display screen by exploiting natural light. Further, to keep abreast with the increasing versatility of data accompanying the ever-continuing advances in multimedia, there is a demand for reflecting-mode liquid crystal display apparatus not only capable of color display and high image-quality (high-definition) display in large screen but also of low production cost.
As a reflective liquid crystal display elements constituting the reflecting liquid crystal display device, there is known a variety of elements such as TN (Twisted Nematic) and STN (Super Twisted Nematic) elements, but elements utilizing a polarizer (one polarizing plate type) is preferred for color display and high-definition display. For example, the R-OCB mode in which the liquid crystal layer is of the HAN (hybrid aligned nematic) alignment has excellent characteristics such as low voltage, wider viewing angle, high-speed response, middle color rendition and high contrast.
In order to insure the uniform brightness of the screen as accompanied with getting a display screen of portable equipment larger, the scattering function is an important factor. That is, in the reflective liquid crystal display device, the brightness of the screen is insured in such manner that the light incident on the liquid crystal layer (natural light, ambient light) is efficiently taken in and reflected with a reflector, and the reflected light is scattered to an extent not deteriorating visibility for the prevention of total reflection. When the polarizer and light-scattering sheet are combined, the reflection efficiency can be further improved. Incidentally, as the reflecting plate (reflector), a light-reflecting back electrode system in which an electrode functions as the reflecting plate and a reflecting plate which is disposed on the outside of a support substrate of an electrode can be employed.
For example, concerning reflective liquid crystal display devices, in Japanese Patent Application Laid-Open No. 228887/1988 (JP-63-228887A) and Photofabrication Symposium ""92 sponsored by the Japanese Society of Printing, the fundamental technology about liquid crystal display device and the liquid crystal display device given an enlarged viewing angle of the display screen through the prevention of total reflection by means of a surface-corrugated metal thin film as the back electrode (lower electrode) were introduced.
However, when the reflective liquid crystal display device is to be a color display, a color filter is used in addition to the polarizer. In case where a color filter is used, the proportion of loss of reflected light is increased and the above diffusion reflecting plate (scattering plate) system cannot impart enough brightness to the display screen. Particularly, in color display devices, it is important that high luminance is imparted by directed-type scattering in which the scattered light is directed in a given direction. However, in order to enhance the directionality by scattering and reflecting plate system, it is necessary that configuration and distribution of uneven part of the reflecting plate is accurately controlled, and it requires a high cost.
For the purpose of insuring a high luminance by scattering reflected light, there is also known a liquid crystal display device with a transmittable light-scattering sheet in lieu of the diffuse reflecting plate. For example, Japanese Patent Publication No. 8430/1986 (JP-61-8430B) discloses a liquid crystal display device comprising a polarizing layer formed on the front side of a liquid crystal cell and, as formed thereon, a light-scattering layer. Moreover, there is also known a resin sheet polymerized utilizing holography to impart directionality to a transmitting type light-scattering sheet (The synopsis of Lectures at Japanese Society of Liquid Crystal Science, 1998). However, in order to impart directionality to a transmitting type light-scattering sheet by polymerization with utilizing the above holography, complicated method of producing, and as a result, raises the production cost is required.
Meanwhile, known as a light-scattering sheet of low production cost is a particle scattering type sheet comprised of plastic beads and a transparent resin constituting a islands-in-an ocean structure. For example, Japanese Patent Application Laid-Open No. 261171/1995 (JP-7-261171A) discloses a display device having a light-scattering layer externally of a liquid cell, specifically a display device comprising a polarizing film on the outer surface of an electrode plate and, as formed on the surface of the polarizing film, a light-scattering layer comprising a phase separated dispersion of two or more kinds of resins varying in refractive index. Japanese Patent Application Laid-Open No. 27904/1995 (JP-7-27904A) and 113902/1997 (JP-9-113902A) disclose a transmittable liquid crystal display device in which a particulate-scattering sheet having an islands-in-an ocean structure composed of a plastic bead and a transparent resin is disposed between a backlight and a liquid crystal cell. As an example of the display device having a light-scattering layer within the liquid crystal cell, Japanese Patent Application Laid-Open No. 98452/1995 (JP-7-98452A) discloses a display device comprising a transparent resin layer containing a dispersed fine particle (the light scattering layer) between an electrode and a substrate (electrode support substrate) of an electrode plate.
However, in these islands-in-an ocean structure sheets, since the resin beads are dispersed randomly in a transparent resin matrix, the scattering light intensity distributes according to Gaussian distribution in principle. Thus, the directionality can not be imparted to the scattered light, and it is difficult that brightness of the display surface is advanced. Particularly, in respect to the particle dispersed sheet, the brightness of the reflected light from a reflector is increased in the reflective liquid crystal display device having a large display screen, so that the sufficient brightness can not imparted to the periphery of the display screen. On the other hand, the brightness is imparted to the whole display screen to some extent, so that the display screen goes dark as a whole and the visibility is lowered. Therefore, it is difficult in the reflective liquid crystal display device having a relatively large display screen such as a reflective liquid crystal display device having 1.5 inch or more display surface area that the whole display screen is illuminated.
It is, therefore, an object of the present invention to provide a light-scattering sheet (or film) capable of imparting high directionality and diffusibility to a transmitted and scattered light, and a liquid crystal display device (particularly, reflective liquid crystal display device) with the light-scattering sheet.
It is another object of the present invention to provide a light-scattering sheet (or film) capable of illuminating the whole screen, and a liquid crystal display device with the light-scattering sheet (particularly, reflective liquid crystal display device).
It is still another object of the present invention to provide a light-scattering sheet capable of imparting the brightness to the display surface even in large display surface, and a liquid crystal display device (particularly, reflective liquid crystal display device) with the light-scattering sheet.
It is other object of the present invention to provide a light-scattering sheet capable of displaying high quality images sharply and brightly even in color display, and a liquid crystal display device with the light-scattering sheet.
It is further object of the present invention to provide a light-scattering sheet capable of displaying image with high luminance and high definition and a liquid crystal display device with the light-scattering sheet.
It is another object of the present invention to provide a process for producing the light-scattering sheet with ease and at low cost.
The inventors of the present invention did much research to accomplish the above objects and found that by causing spinodal decomposition under a suitable condition by evaporating or removing a solvent from a homogenous solution containing a plurality of polymers differing in refractive index to form an isotropically droplet phase structure (phase separation structure) having one or two kinds of regularities for an average interphase distance of the droplet phase, and diffusibility and directionality toward one- or two-direction(s) can be imparted to transmitted and scattered light with the use of the sheet having the regular phase separation structure. The present invention has been developed on the basis of the above findings.
Thus, the transmittable light-scattering sheet of the present invention comprises a light-scattering layer composed of a plurality of polymers varying in refractive index and having a droplet phase structure. The light-scattering layer transmits and scatters (or diffuses) an incident light isotropically. The light-scattering layer has a specific characteristic that a maximum value of a scattered light intensity appears at one or two specific scattering angles. That is, the light-scattering layer comprises (1) a light-scattering layer expressing a maximum value of scattered-light intensity at scattering angle of 3 to 40xc2x0 and (2) a light-scattering layer expressing maximum values of scattered-light intensity at two scattering angles. The latter light-scattering layer has such light-scattering property that maximum values (peaks) of the transmitted and scattered light intensity appear within scattering angle range (that is, the scattering angle range on both sides of the scattering center). In the scattered light intensity, the smaller angle xcex8a having a maximum value may be, for example, about 2 to 20xc2x0, a ratio xcex8b/xcex8a of a smaller angle xcex8a to a larger angle xcex8b having maximum values is, for example, 1.5 to 10. The latter light-scattering layer has at least a droplet or an island-in an ocean phase structure, and a particle size distribution of a dispersed phase in the phase structure has two peaks differing in average particle size. That is, the phase separation structure has a dispersed phase differing in the average particle size, and the layer has two kinds of regularities.
An average diameter of the droplets in the droplet phase structure may be about 0.1 to 20 xcexcm. An average distance between droplet centers in the droplet phase structure may be about 0.5 to 15 xcexcm, and a standard deviation of the average distance between droplet centers (average interphase distance) may be not more than 40% of distance between droplet centers. Further, a volume of the droplets in the droplet phase structure may be about 30 to 70% based on the whole light-scattering layer. A total light transmittance of the light-scattering sheet is about 70 to 100%.
A difference between refractive indexes of a plurality of polymers constituting the light-scattering layer is, for example, about 0.01 to 0.2. Moreover, the plurality of polymers can be selected from, for example, a styrenic resin, a (meth)acrylic resin, a vinyl ester-series resins, a vinyl ether-series resin, a halogen-containing resin, an alicyclic olefinic resin, a polycarbonate-series resin, a polyester-series resin, a polyamide-series resin, a silicone-series resin, a cellulose derivative and a rubber or a elastomer. At least one component of the plurality of polymers may be, for example, cellulose esters (e.g., cellulose acetate). The plurality of polymers can comprise a first polymer and a second polymer, and a ratio of the first polymer to the second polymer may be the former/the latter=10/90 to 90/10 (weight ratio).
Such the light-scattering layer having the phase separation structure may be formed by spinodal decomposition, for example, wet spinodal decomposition from a liquid phase comprising a plurality of polymers. In the wet spinodal decomposition, the plurality of polymers which is non-crystalline and soluble in a common solvent [in which a plurality of resins can be dissolved] can be used.
The light-scattering sheet of the present invention may be a sheet solely comprising the light-scattering layer, and may be a laminated sheet which comprises a transparent support and a light-scattering layer laminated on at least one side of the transparent support. In the laminated sheet, the transparent support is usually substantially isotopic under optics.
In a process of the present invention, the light-scattering sheet is produced by removing or evaporating a solvent from a liquid mixture composed of a plurality of polymers varying in refractive index to form the light-scattering layer having at least a droplet phase structure (or the phase separation structure) due to spinodal decomposition. Such process may comprise applying the liquid mixture on the transparent support and removing a solvent in the mixture to form the light-scattering layer or the phase separation structure.
The present invention includes also a reflective liquid crystal display unit which comprises a liquid crystal cell having a liquid crystal sealed therein, a reflecting means for reflecting an incident light disposed behind the liquid crystal cell, and the light-scattering sheet disposed forwardly of the reflecting means. In the unit, a polarizing plate may be disposed forwardly of the liquid crystal cell, and the light-scattering sheet may be disposed between the liquid crystal cell and the polarizing plate.
Throughout this specification, the term xe2x80x9csheetxe2x80x9d means, without regard to thickness, a dimensional material thus meaning a film as well.